The present invention relates to a valve device for at least one hydraulic motor suitable for driving a mass of large inertia, the motor being designed to be connected to a fluid circuit which includes two main ducts, namely a fluid feed duct and a fluid discharge duct, which ducts are suitable for being closed off when the motor is not operating, and an auxiliary duct in which the fluid pressure is lower than the feed pressure of the motor.
The mass that the motor serves to drive is referred to below as the xe2x80x9cdriven massxe2x80x9d.
The motor to which the valve device applies serves, for example, to rotate a turret on plant such as a hydraulic excavator, or to move in translation plant having tracks or tires of large mass.
It may be a hydraulic motor of the xe2x80x9cfast motorxe2x80x9d type (1000 to 2000 revolutions per minute (r.p.m.)) driving gearing, or a xe2x80x9cslow motorxe2x80x9d (whose rotary speed is about 100 r.p.m., for example), e.g. of the type having radial pistons.
In operation, a flow of fluid is maintained through the motor, the main ducts being connected to main orifices of the motor (serving for feed and for discharge), so that one of the main ducts is put under pressure so as to act as feed duct, while the other of the ducts is at a relatively lower pressure and is connected to fluid removal means so as to act as discharge duct.
Starting from a situation in which the motor is operating at a given drive speed, the motor is stopped by performing a deceleration stage, and then by closing off the feed and discharge ducts. During the deceleration stage, the pressure in the feed duct becomes low pressure, while the pressure in the discharge duct becomes high pressure. Finally, on closing off the main ducts of the motor, i.e. on isolating the motor, the fluid situated in the discharge duct is at a pressure that is higher than the pressure of the fluid situated in the feed duct. This phenomenon is further reinforced by the fact that, due to its large inertia, the driven mass tends to continue its initial movement.
On flat terrain, the system reaches equilibrium only when the pressures in the feed and discharge ducts are substantially equal. On sloping terrain, or when the driven mass is leaning, the system reaches equilibrium only when the difference between the pressures in the feed and discharge ducts reaches a given value (positive or negative) that makes it possible to compensate for the slope in order to hold the mass stationary.
In any event, in order for the motor and the driven mass to be actually stopped in a stable position, it is necessary for the pressure difference between the feed and discharge ducts to reach a given value, be it zero, positive, or negative.
It is indicated above that, on closing off the feed and discharge ducts, the discharge duct is at high pressure that is further increased by the inertia of the driven mass. This high pressure tends to push back the driven mass in a return movement in the opposite direction, thereby transferring to the feed duct (which is closed off) the high pressure of the discharge duct (which is also closed off).
In addition, the hydraulic fluid is slightly compressible. As a result, after the motor has been isolated, the inertia mass continues to move until the pressure in the discharge duct reaches a maximum value corresponding to the fluid present in said discharge duct being compressed. The return movement of the mass causes the pressure in the feed duct to increase until the fluid present in said feed duct is brought to a compression pressure that is substantially equal to the maximum pressure that prevailed in the discharge duct just before the return stage began.
Naturally, the return stage is followed by another movement stage in the initial direction, during which expansion takes place in the feed duct and compression takes place in the discharge duct.
Thus, after closing off the feed and discharge ducts, an oscillating movement is imparted to the driven mass, the frequency of oscillation for turrets of plant such as hydraulic excavators being about 1 Hz. Although the oscillating movement is of relatively low amplitude, and is finally braked naturally due to friction phenomena, it is clearly extremely inconvenient, in particular when the mass driven by the motor is to be placed in a very precise position by stopping the motor without mechanical braking.
Paradoxically, the oscillating motion phenomenon used to be less of a problem when the drive was provided by means of low-performance motors in which the relatively large leaks limited the compression in the feed and discharge ducts. Motors have gradually been perfected, in particular to improve efficiency, to reduce the duration of the acceleration stage, and to facilitate handling in difficult conditions, e.g. when leaning.
To limit the oscillations, i.e. to reduce their amplitude and finally to stop them, it is known that a damping system can be used, consisting in creating leaks between the feed and the discharge ducts, which leaks feed a transfer volume. After isolating the motor, it is possible to compensate, at least partially, for the pressure difference between the feed and discharge ducts by using the fluid available in the transfer volume.
Another system consists in allowing continuous leaks to take place between the feed and discharge ducts of the motor.
Those systems are not fully satisfactory insofar as they result in a reduction in the efficiency of the motor, in contradiction with the efforts that have been made to increase efficiency, and insofar as they make it almost impossible to position the driven mass accurately on stopping the motor. For example, when the motor serves to drive the turret of a hydraulic excavator, the turret actually stops with an angular offset relative to the target angular position in which the motor is to be isolated, the angular offset corresponding to the amount of fluid that is available in the transfer volume for being put into circulation.
EP-A-0 457 913 discloses a device seeking to prevent cavitation phenomena and to limit or reduce shocks when the motor driving a mass of large inertia is stopped.
The device comprises a valve which includes first and second main ducts serving to be connected to respective ones of the two main ducts of the fluid circuit, and an auxiliary duct serving to be connected to the auxiliary duct of said circuit, the device being able to have a first configuration in which, with the fluid pressure in the first main duct being greater than the fluid pressure in the second main duct, said second main duct is connected to the auxiliary duct, while the first main duct is isolated from the second main duct and from the auxiliary duct, and a second configuration in which, with the fluid pressure in the second main duct being greater than the fluid pressure in the first main duct, said first main duct is connected to the auxiliary duct while the second main duct is isolated from the first main duct and from the auxiliary duct.
The valve enables the main duct, which is at a low pressure when the motor is stopped, to be connected to the auxiliary duct, thereby preventing cavitation in said main duct. In other words, the valve serves only to select the duct which is at low pressure and to connect it to a booster pressure.
The device does not however prevent almost unbraked oscillations from occuring after the motor has been stopped. As a function of said oscillations, the valve passes alternately from its first configuration to its second configuration without rapidly limiting the pressure difference between the two main ducts.
An object of the invention is to remedy the above-mentioned drawbacks by providing a device that is simple and reliable, and that makes it possible to brake and to reduce to zero very rapidly the oscillations of the system after isolating the motor, regardless of the conditions under which the mass is being driven, in particular regardless of whether it is being driven on sloping or banking terrain, or on flat terrain.
This object is achieved by the fact that the valve device includes time delay means suitable for limiting the speed of transition between one and the other of the first and second configurations when the difference between the pressures in the first and second main ducts changes sign.
To explain how the device operates, it is considered, for example, that the first main duct of the device is connected to the discharge duct, and that the second main duct is connected to the feed duct.
As indicated above, on decelerating and then stopping the motor (closing off the main ducts), the fluid pressure in the discharge duct is greater than the fluid pressure in the feed duct. Therefore, during the deceleration stage and until the motor stops, the valve device of the invention remains in its first configuration, the feed duct being brought to the pressure of the auxiliary duct.
Because of its large inertia, the driven mass continues its initial movement until the pressure in the discharge duct reaches a maximum value (compression). Starting from this situation, the driven mass begins a return movement during which the difference between the pressures in the discharge and feed ducts decreases until it is substantially zero, and it then changes sign.
When the pressure difference reaches zero, the mass continues its movement under the effect of its inertia, which tends to cause the pressure in the feed duct to increase by compression, and to cause the pressure in the. discharge duct to decrease.
When the pressure difference changes sign, the device is urged towards its second configuration in which the feed duct is isolated while the discharge duct is brought to the pressure of the auxiliary duct.
If the time delay means were not provided, the device would go from its first configuration to its second configuration too quickly, and the feed duct, as isolated as soon as the pressure difference between the feed and discharge ducts changes sign, would undergo a rapid and/or strong increase in pressure by compression, due to the movement of the driven mass until the end of oscillation thereof, and then the movement of the mass, generated by the high pressure in the feed duct, would start again in the opposite direction, and so on, so that a regime of almost unbraked oscillations would be set up.
By the presence of the time delay means, it is guaranteed that, during a given xe2x80x9ctime delayxe2x80x9d time, the device remains in its first configuration in spite of the change in sign of the pressure difference between the feed and discharge ducts. During the time delay time, the increase in the pressure in the feed duct is very limited, because the second main duct of the device (the main duct that is connected to the feed duct) remains in communication with the auxiliary duct.
Thus, at the end of the return movement of the driven mass, the pressure in the feed duct is, at the most, only very slightly greater than the pressure in the discharge duct, so that the driven mass is urged to undergo at the most one more movement in the opposite direction (in the xe2x80x9cgoxe2x80x9d direction again) and of low amplitude, during which movement, when the pressure difference between the feed and discharge ducts changes sign again, the time delay means play their part once again by limiting the increase in pressure in the discharge duct. It is possible to determine the time delay means such that the pressures in the feed and discharge ducts are substantially equal as of the end of the return movement, so that the one more movement in the go direction does not occur.
Thus, by using the time delay means, the oscillations of the driven mass experience a very rapid decrease in their amplitude until the mass stops completely.
In a particularly advantageous embodiment, the device includes a moving member suitable for being urged between two end positions as a function of the difference between the fluid pressures prevailing in the first and second main ducts, and, in its first and second end positions, said moving member puts the second main duct or the first main duct as the case may be in communication with the auxiliary duct via a calibrated constriction passageway.
Thus, the valve device is formed simply, the two end positions of the moving member corresponding to the above-mentioned first and second configurations.
The presence of the calibrated constriction passageway causes head loss between the auxiliary duct and that one of the main ducts to which it is connected, the head loss making it possible to ensure that the pressure does actually increase in the main duct despite it being connected to the auxiliary duct. Thus, the pressure in said main duct can increase until it becomes greater than the pressure in the other main duct which, to begin with, was at a higher pressure, so that the change in sign of the pressure difference that causes the moving member to move towards its other end position can occur.
Advantageously, the moving member is suitable for taking up a third position referred to as the xe2x80x9cintermediate positionxe2x80x9d. In a first variant, the main ducts and the auxiliary duct are isolated from one another in the intermediate position, whereas, in a second variant, they are connected to one another in said intermediate position.
In a particularly advantageous embodiment, the moving member is formed by a slide mounted to slide in a bore to which the two main ducts are connected, a communication duct connected continuously to the auxiliary duct being connected to said bore, in a portion of said bore that extends between the connection zones in which the two main ducts are connected to the bore. The slide has means (e.g. an annular groove) for establishing communication selectively, which means establish a link between the second main duct and the communication duct when the slide is in its first end position, and establish a link between the first main duct and said communication duct when the slide is in its second end position, the fluid flowing in said links going through calibrated constriction means. The slide also has closure means (e.g. regions of its axial peripheral wall in which the groove does not extend) which isolate the first main duct from the communication duct when the slide is in its first end position, and which isolate the second main duct from the communication duct when the slide is in its second end position.
Advantageously, the slide is moved between its two end positions under the effect of an increase in the fluid pressure in a first control chamber or in a second control chamber, the first and second control chambers being connected continuously to respective ones of the first and second main ducts.
The time delay means advantageously comprise first and second damping chambers situated at respective ones of first and second ends of the slide, the first and second damping chambers being continuously in communication with the auxiliary duct via calibrated constriction means that are calibrated to hinder the flow of fluid at least in the direction in which the chambers are emptied.